I just went back and re-read the OP. Not a whole lot on show attire, there!
In Western Riding the preferred method would be one-handed. That's the way a cowboy would have ridden, so that he one hand free to work his rope or any other tool he needed to use from horseback.
As to "Show Attire" just what kind of show are we looking at? What type of classes will be entered? At what level are you showing (a "Saturday Night Horseshow" or a regional or national show)? Some shows are sticklers for the Rules and some are not.